It is often supposed that in order to refute the view that laws of nature are necessary truths it is sufficient to appeal to Hume's argument from the conceivability of to the possibility of their being false. But while Hume's argument does present the necessitarian with insuperable difficulties it needs to be made clear just what these are. The mere appeal to Hume is quite insufficient for what he says can be interpreted in more than one way. And if it constitutes an argument rather than a mere assertion Kneale has given reason to suppose that it is at least not obviously valid. The upshot of this article is that Hume's argument may be seen as a direct challenge to the notion that there could be propositions whose modal value is necessarily "opaque to the human intellect".